ABOARD THE EMPIRE BUILDER, May 28 (Reuters) - Its passengers are mostly silver-haired retirees, oil-field workers and a few young families gazing out the windows of Amtrak's least-profitable and third-longest line, rumbling from Chicago through eight states and on to the American West Coast.

As the Empire Builder snakes along 2,230 miles (3,590 km) on a 46-hour journey, it offers insights into the financial problems plaguing America's passenger railroad at a time of intensifying scrutiny over its aging infrastructure and safety record.

Following Amtrak's May 12 fatal derailment in Philadelphia, the 86-year-old line sums up its problems - and its promise. Running most of its length near no major highways, the Empire Builder reaches into some of America's most rugged, remote and rural regions. A coach ticket between Spokane, Washington and Williston, capital of North Dakota's oil boom, costs $111, much cheaper than flying.

Its supporters include both Republican and Democrat politicians, illustrating how the often-vicious partisan rancor over Amtrak in Washington evaporates in sparsely populated areas of the country that need it most.

And like many of Amtrak's long-distance routes, it's growing.

The shale boom has helped swell passenger numbers, which are up 28 percent between 1997 and 2014.

The increase comes from workers like John Dirby, who rides the Empire Builder from his Montana home to North Dakota's oil fields, where he works as a truck driver, a 10-hour trip. It's longer than by car but worth it, he says. "I spend tons of time for that job driving already. Why would I want to add on more time behind a wheel?"

But the demand isn't enough to make it financially viable: the service lost $34.8 million between October and March, more than the same period in the previous fiscal year when it lost $31.9 million.

Many seats remain empty. When a Reuters reporter traveled from St. Paul, Minnesota to Williston, several sleeper cars were vacant at various intervals. In the coach cabin each passenger had at least a two-person seat to themselves for most of the trip.

To pay for its money-losing routes, Amtrak peels funds away from its profitable ones such as the Northeast Corridor, where its high-speed Acela and Northeast Regional trains saw a combined $484.7 million profit in its last fiscal year.

That's stoking debate over the future of Amtrak and how far Washington should foot the bill for upgrades to aging infrastructure and safety technology at the federal railroad that has bled red ink since its 1971 creation. Of its 48 lines, only five make a profit and one breaks even.

Democrats have called for $1 billion in additional funding for Amtrak. Chuck Schumer, a Democratic Senator from New York, says such investments could have prevented the Philadelphia accident, citing Amtrak's failure to install Positive Train Control on the sharp bend where the train derailed. The mechanism, federal safety officials say, would have automatically slowed the speeding train.

Many Republicans, backed by conservative groups including Club for Growth, say the federal passenger rail authorization spending of $7.2 billion over four years is already too much. The Republican-controlled House Appropriations Committee voted on May 13, a day after the crash, to approve a $1.14 billion Amtrak budget for next fiscal year, a $252 million cut from this year.

John Mica, a Republican in the House, urged privatization for Amtrak as a solution to what he called a "third-world rail system ... run in a Soviet-style operation."

Caught in the middle are heartland politicians, including some Republicans, who want to preserve Amtrak services to their remote towns, along with an eclectic mix of passengers.

"If one can afford the time, this is a fun, carefree way to travel," said Mary Firestine, 60, who took the Empire Builder's entire route from Chicago to connect with a ferry in Seattle en route to an Alaskan job.

TENSIONS

The push to roll back Amtrak funding has laid bare tensions between Washington's Republican establishment and North Dakota's Republican-dominated Congressional delegation, a fight that echoes tensions elsewhere in the country.

"I will fight to at least maintain current funding for Amtrak," Representative Kevin Cramer, North Dakota's Republican congressman, said in an interview. "Clearly the role of Amtrak in North Dakota has changed from one of convenience and local economic development to an essential form of transportation for a workforce highly in demand."

Other long-distance routes are also growing, while losing millions of dollars. The Southwest Chief, from Chicago to Los Angeles, recorded a 10.5 percent increase in ridership in the year to March, a period when it lost $30 million. The number of passengers on the Sunset Ltd, from Louisiana to California, rose 14.5 percent while it lost $18.1 million, according to Amtrak data.

Eighteen states contribute funds to Amtrak, including nine led by Republican governors.

"All of the rhetoric over Amtrak that exists in D.C. simply does not exist outside D.C.," said Robert Puentes, a senior fellow at the Brookings Institution who specializes in transportation and infrastructure. "It's a completely different conversation."

In total, long-distance routes carry about 14.6 percent of Amtrak's passengers but soak up about 41 percent of the costs.

Those who board the Empire Builder have noticed some cost-cutting recently, including the loss of a free wine tasting program the rail network discontinued after Amtrak's inspector general in 2013 called the practice too expensive.

"I miss the wine and cheese nights," Theo Anderson, a retired banker from Seattle, said over a plate of eggs in the train's dining car as the North Dakota prairie retreated behind him. "I guess they're trying to save money." (Additional reporting by David Lawder in Washington. Editing by Jason Szep and Martin Howell)